Micromirrors formed in semiconductor wafers are used in many devices, such as, for example, medical imaging devices, optical spectrometers and also in barcode readers.
Herein we will consider micromirrors formed from portions of a monocrystalline semiconductor substrate, usually in silicon, fixed to the substrate and capable of being orientated with respect to the substrate by means of thermally deformable arms.
FIGS. 1 and 2 illustrate, in plan view and in cross-section taken in the plane A-A′, a micromirror 1 as described in the article “Thermally actuated micro scanner for bar code reader application”, by F. Khechana, H. Van Lintel, J.-L. Massieu, S. Ackley and P. Renaud, published in Optical MEMS conference 2005.
Micromirror 1 is fixed to a silicon wafer 3 by thermally deformable arms 5. Arms 5 extend, in plan view, in a comb shape between wafer 3 and the micromirror 1. The arms 5 are formed of a portion of continuous conductive track 9, which extends between conductive contacts 20 and 21 deposited on the wafer 3. The conductive track 9 rests on an underlying insulating layer 7. For example, the separation between the wafer 3 and the micromirror 1 is between 20 and 40 μm, and the thickness of micromirror 1 is between 20 and 100 μm. Micromirror 1 has the shape of a square having sides of one millimetre in length. While a voltage is applied between the two contacts 20 and 21, the conductive line 9 heats up, and the arms 5 deform by a bimorph effect which causes the micromirror 1 to move as illustrated by the arrows in FIG. 2. The arms 5 are pulled down more when the applied voltage is higher. By regulating the applied voltage, it is thus possible to modify the orientation of the mirror 1. The materials of layers 7 and 9 are also chosen to provide a desired amplitude of displacement of mirror 1. The insulating layer 7 is for example formed of silicon oxide and the conductive line 9 of aluminum.
FIGS. 3A to 3E are partial and schematic cross-section views taken in the plane A-A′ of FIG. 1 and illustrating successive steps in a method of fabricating the micromirror of FIGS. 1 and 2.
As illustrated in FIG. 3A, we start with a monocrystalline silicon wafer 3 having a thickness in the order of 300 to 400 μm. A layer 7 of silicon oxide is formed on the two sides of the wafer 3. The layer 7 has a thickness of around 1 μm.
An aluminum layer 9 is deposited on the front and back faces of wafer 3. The layer 9 has a thickness of around 1 μm.
In the next steps, the result of which is illustrated in FIG. 3B, on the front face, the stack of layers 9 and 7 is partially removed so as to expose the wafer 3. The partial removal is performed by means of a mask 40 illustrated in plan view in FIG. 4. The plane A-A′ of the cross-section of FIG. 3B is shown in FIG. 4. The removal is performed such that only the stacks 7-9 providing the shape of the arms 5 in FIGS. 1 and 2 remain. The arms 5 have a width of between 5 and 40 μm, preferably around 30 μm. The contacts 20 and 21 can have sides of 100 μm or more in length. Two arms are separated by a gap having the same order of magnitude as the width of the arms. As illustrated in FIG. 3B, on the back face, layer 7 is removed such that a central part of the wafer 3 is exposed.
Next, as illustrated in FIG. 3C, a mask 50 is deposited on the front face. FIG. 5 illustrates in plan view the mask 50. The plane A-A′ of the cross-section of FIG. 3C is shown in FIG. 5. The mask 50 corresponds to the striped region in FIG. 5. It comprises a central square 52 linked by arms 54 to a frame 56. The central square corresponds to micromirror 1 and the arms 54 to the arms 5 formed previously.
As illustrated in FIG. 3D, etching is then performed on the back face, of the silicon forming the wafer 3. The etch is a wet etch performed with the help of potash (KOH). Etching occurs on back face everywhere where the back face is not protected by the mask provided by insulating layer 7. The etch is continued until, with regard to the opening of layer 7, only a thin layer of wafer 3 remains, in the order of 30 to 40 μm, on the front face of wafer 3. This thin layer corresponds more or less to the desired thickness of the micromirror.
As illustrated in FIG. 3E, we then proceed with an isotropic etch from the front face. The surfaces of wafer 3 not protected by the mask 50 are etched. The micromirror 1 is thus delimited and only remains linked to the rest of the wafer 3 by arms 5. While not visible in the cross-sections, it should be noted that, during these steps, the regions between the arms 5 are removed by etching. Simultaneously, the thin portions of the silicon wafer under the arms 5 are etched, laterally, and are totally removed while the thin portion of wafer which corresponds to the micromirror 1 is only lightly etched on its large sides. The etch is for example a dry etch performed by means of a plasma containing sulfur hexafluoride (SF6).
FIG. 6A illustrates the same structure in plan view. The plane A-A′ of FIG. 6A is the axis of the cross-section of FIG. 3E. FIG. 6B is an enlarged view of a portion of FIG. 6A at the level of frame 60, in other words at the level of the edge between wafer 3 and the region separating wafer 3 from the micromirror. The elements drawn by dashed line in FIGS. 6A and 6B will be described below.
The micromirrors obtained according to the above known method present problems. In particular, it has been found that after a certain number of uses or following shock, cracks in the arms result.
A further problem with these micromirrors is the appearance of errors in the orientation of the mirror during the lifetime of the device.
A further problem with these micromirrors is the appearance of defects in the form of cracks in the arms at their fulcrums on the side of the mirror and on the side of the wafer.
A further problem with these micromirrors is the fact that the previous problems are not detectable during test phases but appear during use of the device containing the micromirrors, causing breakdown of the device and imposing particularly difficult maintenance operations on the user.